Nonwoven webs with electret properties are known in the art.
A typical method and apparatus for making a nonwoven fibrous electret web is disclosed in WO 1996/026,783 A1. In this method, one or more free-fibers are formed from a nonconductive polymeric fiber-forming material and exposed to an AC and/or DC corona-discharging device. Charges are placed on the fibers and additives, such as organic amide compounds, to slow down the process of decaying over time or in high ambient temperatures.
Further, a method to create an electret is described in WO-A-01/27,371. In this method one or more free-fibers are formed from a nonconductive polymeric fiber-forming material. An effective amount of water is sprayed onto said free-fibers, and the fibers are collected to form a nonwoven fibrous web. The nonwoven web so obtained is dried to form a nonwoven fibrous electret web.
It is observed, however, that it has been known for a long time that electrical charges can be created by means of a waterfall. In 1892, Philipp Lenard already published the first comprehensive paper about the electricity of waterfalls (see for instance Atmos. Chem. Phys., 7, 2271-2275, 2007). Lenard already taught that electric charges can be created by spraying deionized water thereby forming large and small electrically charged droplets. Hence, the skilled person already understood at an early stage that water quenching steps applied to free fibers or a formed nonwoven web would form electric charges on the surface of the water quenched free fibers or nonwoven webs.